The present invention relates to a charge storage device and a method of manufacture thereof.
The invention has been developed primarily for use with the electrochemical charge storage devices such as supercapacitors and will be described hereinafter with reference to that application It will be appreciated that supercapacitors are designated by terms such as ultra capacitors, electric double layer capacitors and electrochemical capacitors, amongst others, all of which are included within the term “supercapacitor” as used within this specification.
It is known to mass produce supercapacitors that have specific operational characteristics that fall within well defined ranges. Although mass production is advantageous from a cost point of view, there is an inherent lack of flexibility. That is, if the desired characteristics of a supercapacitor for a particular application fall outside the commonly available ranges a compromise solution is required An alternative is to produce the desired supercapacitor as a one off or small run. The costs of this latter alternative are often prohibitive and, as such, rarely pursued.
Known supercapacitors generally find application in power supplies such as uninterruptible power supplies for computers or backup power supplies for volatile memory. Accordingly, it has been common to optimise these supercapacitors for high energy density, low self-discharge rates, and low cost.
More recently it has been thought that supercapacitors are theoretically applicable to high power pulsed applications. Indeed, some attempts have been made to adapt such supercapacitors as short term current sources or sinks. Examples of such applications include internal combustion engine starting, load power leveling for hybrid vehicles and a variety of pulsed communication systems. However, the success of these supercapacitors has been limited by factors such as a high equivalent series resistance, among others. For example, some prior art double layer capacitors make use of button cell or spiral wound technology. These, in turn, fall generally in one of two groups, the first group being concerned with high power applications and the second with low power applications. For the second group, but not the first, it has been possible to obtain high energy densities.
The first and second groups are broadly defined by the type of electrolyte used, those being aqueous and non-aqueous respectively. This is predominantly due to the lower resistance inherently offered by aqueous electrolytes which makes it better suited to high power, and hence high current, applications. That is, the low resistance results in lower I2R losses for aqueous electrolytes. The trade off, however, is that for these aqueous electrolytes the voltage that can be applied across a capacitive cell is extremely limited.
The second group of prior art double layer capacitors suffers the converse disadvantages. That is, while they provide a greater voltage window, which improves the available energy density, they also have had high internal resistances which make them unsuitable to the high power applications.